The present invention generally relates to communication line backup systems, and more particularly to a communication line backup system which can establish a backup line between subscriber terminals via a communication switching network when a failure occurs on a communication line.
In the conventional telephone and telegraph switching network and in the integrated services digital network (ISDN) which may possibly replace the conventional telephone and telegraph switching network, a failure may occur on a leased line or a normal line which is used between subscribers. In order to continue the communication even after the failure occurs, a backup line must generally be provided within the communication network.
FIG. 1 shows an example of a conventional communication line backup system. In FIG. 1, subscriber terminals 102 and 104 are coupled via a leased line 106. A modem 108 is provided between the subscriber terminal 102 and the leased line 106, and a modem 110 is provided between the subscriber terminal 104 and the leased line 106. This backup system uses a telephone switching network 112.
However, even when a telephone line of the telephone switching network 112 is used as the backup line between the subscriber terminals 102 and 104, the telephone line which is used for the backup is not always the same. For this reason, the connection route within the telephone switching network 112 is established depending on the congestion of the telephone switching network 112.
If different telephone lines are established for the backup, the line quality varies depending on the telephone line which is used as the backup line. This variation in the line quality affects the communication as the transmission rate of the modems 108 and 110 becomes high, on the order of 19.2 kbps, for example. Accordingly, it is difficult to provide a stable backup connection using the telephone switching network 112 in which the line quality deterioration described above occurs.
On the other hand, it is conceivable to use the ISDN for the backup system. However, in a communication switching network in which the existing telephone switching network and the ISDN coexist, a telephone line backup unit which is used in the existing telephone switching network cannot be connected to an ISDN backup unit which backs up ISDN terminals within the ISDN. For this reason, it becomes necessary to use two kinds of backup systems, that is, the backup system for the telephone line and the backup system for the ISDN. As a result, the maintenance and management of the communication in such a communication switching network becomes complex.